With the development of medical technologies, medical scanning is becoming increasingly popular as an important diagnostic and treatment tool in many medical applications. For example, computed tomography (CT) has been widely used in diagnostic tests and radiotherapy for patients. In a CT system, an x-ray source projects a fan-shaped beam which is collimated to be within an X-Y plane of a Cartesian coordinate system, generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. Then, the intensity of the attenuated x-ray beam is detected by the detector array to thus construct an x-ray image (i.e., CT image).
In the widely used third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In CT scanning, since the x-ray source projects a fan-shaped beam, the fan beam data is acquired instead of parallel beam data. However, the theory of imaging reconstruction was initially developed for parallel beam data. Therefore, one important step in CT imaging reconstruction is to rebin data from the fan beam data into parallel beam data to thus convert the fan beam projection data into the parallel beam projection data. By the data conversion, a set of fan beam detector samples taken during a scan by the CT system is converted into a set of equivalent parallel detector samples. This eliminates the distance weight otherwise required in fan beam reconstruction. Without the conversion to parallel beam projection data, the distance weight may cause a “zebra” or banding artifact across Z direction in reconstructing three dimensional data. Thus, data conversion from fan beam data to parallel beam data in the imaging reconstruction allows for a more direct and accurate implementation of reconstruction algorithms.
In third generation CT systems, all detector cells are uniformly distributed along the same arc whose center is the x-ray source. Currently, conversion from fan beam data to parallel beam data only focuses on the third generation curved detector. For example, the U.S. Pat. No. 6,411,670 patent describes a method for generating an enhanced object CT image, comprising rebinning the fan beam projection data obtained by the curved detector array with equal γ angle into parallel beam projection data. All contents of this patent will be incorporated into the present application by reference.
Other prior art related to fan-to-parallel data rebinning includes U.S. Pat. No. 4,570,224, U.S. Pat. No. 5,216,601, U.S. Pat. No. 4,852,132, and etc. In addition, the following publications also involve fan-to-parallel data rebinning for a curved detector array, i.e., “Rebinning-based algorithms for helical cone-beam CT, 2001 Phys. Med. Biol. 46” and “Advanced single-slice rebinning in cone-beam spiral CT, Med. Phys. 27, April 2000”. These patents and papers also are incorporated into the present application by reference.
The prior art to some extent alleviated the problem of applying fan beam projection data to CT image reconstruction. However, the prior art is inapplicable to the large flat module detector array, which has cost advantages. Different from traditional rebinning algorithms based on equal gamma (γ) angle geometry, the large flat module detector array has unequal γ angle geometry. Applying the traditional rebinning algorithms to the flat module detector array may lead to incorrect rebinning results, and further causes severe ring artifacts.
There still exist other defects regarding fan-to-parallel data conversion in the prior art. Thus, it is desired that a new technical solution can improve the prior art in one or more aspects. For example, the new technical solution is expected to eliminate ring artifacts in use of the flat module detector array.